Recently, a service of operating a virtual machine (VM) on a server and offering it to a user is delivered in a system configuration which applies cloud computing or thin clients. On such a server, one or more VMs operate, and a physical resource (for example, a central processing unit (CPU), a memory, etc.) that is virtualized by software is allocated to each VM. As a method for respectively allocating a physical resource to VMs which operate on a single server, there has been proposed a technique to dynamically allocate a physical resource to the respective VMs which operate on a single server according a state in which the physical resource of the server is used by the respective VMs.
On the other hand, for a system configuration in which two or more servers are present, the following technique is adopted so that VMs can normally operate on each server. That is, the technique is a VM arrangement method, in which a permitted available range of each server is set by determining an upper limit and a lower limit of a ratio of a physical resource capacity used by VMs in each single server with respect to a total physical resource capacity of the server, and the VMs are arranged so that the ratio above falls within the permitted available range. Specifically, if a ratio of a physical resource capacity used by the VMs of any single server with respect to a total physical resource capacity of the server is greater that an upper limit of the permitted available range, one or more VMs are migrated to another server. As a result, even when a load imposed by a VM sharply increases, processing performance degradation of the VM due to shortage of the physical resource can be prevented (see, for example, Japanese Laid-open Patent Publication No. 2002-202959).
With the aforementioned technique, however, VMs may not be efficiently consolidated.